Exported Typeinfo For Pure Virtual Class In A Shared Library

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Introduction

When working with shared libraries in C++, it's essential to understand how typeinfo is handled, especially when dealing with pure virtual classes. In this article, we'll explore the concept of exported typeinfo for pure virtual classes in shared libraries, using Clang as our compiler.

Background

In C++, typeinfo is a fundamental concept that provides information about the type of an object. This information is crucial for various purposes, such as dynamic casting, exception handling, and debugging. When a class is defined, the compiler generates a typeinfo object that contains metadata about the class, including its name, base classes, and virtual functions.

Pure Virtual Classes and Shared Libraries

A pure virtual class is a class that contains at least one pure virtual function. A pure virtual function is a function that is declared but not defined in the class. When a class is declared as pure virtual, it cannot be instantiated on its own and must be inherited by another class.

Shared libraries, on the other hand, are libraries that can be loaded into a program at runtime. They are used to share code between multiple programs, reducing code duplication and improving modularity.

The Problem with Exported Typeinfo

When a shared library contains a pure virtual class, the typeinfo for that class is not exported by default. This means that when a program links against the shared library, it will not have access to the typeinfo for the pure virtual class.

The Test Case

To demonstrate this issue, I created a test case on GitHub: https://github.com/KapitanPL/inside_cast_example. This test case consists of a shared library that contains a complex hierarchy of classes, most of which are virtual.

Compiling the Test Case

To compile the test case, I used Clang as my compiler. I created two separate compilation units: one for the shared library and one for the executable.

Shared Library Compilation

For the shared library compilation, I used the following command:

clang++ -shared -fPIC -o libexample.so example.cpp

This command tells Clang to compile the example.cpp file into a shared library called libexample.so.

Executable Compilation

For the executable compilation, I used the following command:

clang++ -o example example.cpp -L. -lexample

This command tells Clang to compile the example.cpp file into an executable called example, linking it against the libexample.so shared library.

The Issue with Exported Typeinfo

When I ran the executable, I encountered an issue with exported typeinfo. Specifically, when I tried to use the dynamic_cast operator to cast a pointer to a base class to a pointer to a derived class, I received a runtime error.

Debugging the Issue

To debug this issue, I used the Clang debugger to inspect the typeinfo for the pure virtual class. I discovered that the typeinfo was not being exported by the shared library, which was causing the runtime error.

Fixing the Issue

To fix this issue, I needed to explicitly export the typeinfo for the pure virtual class. I did this by adding the following attribute to the class definition:

__attribute__((visibility("default"))) class Base {
public:
    virtual void foo() = 0;
};

This attribute tells Clang to export the typeinfo for the Base class by default.

Conclusion

In conclusion, exported typeinfo for pure virtual classes in shared libraries can be a challenging issue to debug. By understanding the concept of typeinfo and how it's handled in shared libraries, developers can avoid common pitfalls and ensure that their code works correctly.

Best Practices

To avoid issues with exported typeinfo, follow these best practices:

  • Use the __attribute__((visibility("default"))) attribute to explicitly export typeinfo for pure virtual classes.
  • Use the dynamic_cast operator with caution, as it can lead to runtime errors if the typeinfo is not exported correctly.
  • Use the Clang debugger to inspect typeinfo and diagnose issues with exported typeinfo.

Future Work

In future work, I plan to explore other issues related to exported typeinfo in shared libraries, such as:

  • How to handle multiple inheritance and virtual inheritance in shared libraries.
  • How to use typeinfo to implement advanced features, such as polymorphic containers and type-safe function pointers.

Introduction

In our previous article, we explored the concept of exported typeinfo for pure virtual classes in shared libraries using Clang as our compiler. We discussed the issue of exported typeinfo and how to fix it by explicitly exporting the typeinfo for the pure virtual class.

In this article, we'll answer some frequently asked questions (FAQs) related to exported typeinfo for pure virtual classes in shared libraries.

Q: What is exported typeinfo?

A: Exported typeinfo is the metadata about a class that is made available to other parts of a program. This metadata includes the class's name, base classes, and virtual functions.

Q: Why is exported typeinfo important?

A: Exported typeinfo is important because it allows other parts of a program to use dynamic casting, exception handling, and debugging. Without exported typeinfo, these features may not work correctly.

Q: What is the issue with exported typeinfo in shared libraries?

A: The issue with exported typeinfo in shared libraries is that the typeinfo for pure virtual classes is not exported by default. This means that when a program links against a shared library, it will not have access to the typeinfo for the pure virtual class.

Q: How do I fix the issue with exported typeinfo?

A: To fix the issue with exported typeinfo, you need to explicitly export the typeinfo for the pure virtual class. You can do this by adding the __attribute__((visibility("default"))) attribute to the class definition.

Q: What is the __attribute__((visibility("default"))) attribute?

A: The __attribute__((visibility("default"))) attribute is a Clang-specific attribute that tells the compiler to export the typeinfo for a class by default.

Q: Can I use other compilers to fix the issue with exported typeinfo?

A: Yes, you can use other compilers to fix the issue with exported typeinfo. However, the syntax and attributes may vary depending on the compiler.

Q: How do I use the Clang debugger to inspect typeinfo?

A: To use the Clang debugger to inspect typeinfo, you can use the info type command to display the typeinfo for a class.

Q: What are some best practices for working with exported typeinfo?

A: Some best practices for working with exported typeinfo include:

  • Using the __attribute__((visibility("default"))) attribute to explicitly export typeinfo for pure virtual classes.
  • Using the dynamic_cast operator with caution, as it can lead to runtime errors if the typeinfo is not exported correctly.
  • Using the Clang debugger to inspect typeinfo and diagnose issues with exported typeinfo.

Q: What are some common pitfalls to avoid when working with exported typeinfo?

A: Some common pitfalls to avoid when working with exported typeinfo include:

  • Failing to export typeinfo for pure virtual classes, which can lead to runtime errors.
  • Using the dynamic_cast operator without checking if the typeinfo is exported correctly.
  • Not using the Clang debugger to inspect typeinfo and diagnose issues with exported typeinfo.

Conclusion

In conclusion, exported typeinfo for pure virtual classes in shared libraries can be a challenging issue to debug. By understanding the concept of typeinfo and how it's handled in shared libraries, developers can avoid common pitfalls and ensure that their code works correctly.

Additional Resources

For more information on exported typeinfo and shared libraries, you can refer to the following resources:

By following these best practices and avoiding common pitfalls, developers can write more robust and maintainable code that takes advantage of the benefits of shared libraries.